Store the value of display_cal parameter in the session.

Store the value of display_cal parameter in the session.

This makes display_cal do what it was originally inteneded to do.
Does not update user prefs (that's what toggle_calendar is for), but
just makes sure that ONLY the calendar(s) in the parameter are displayed.
Must pass an empty value for display_cal to clear the session value.

Still need to work out how to trigger the emptying of the value...probably
will replace the calendar panel with a 'close' link or similar when the session
variable contains data. This also solves the issue of dealing with the user changing
the calendars displayed via the panel while showing only display_cal.